The present invention relates to an injection moulding assembly and a mould therefor and, more specifically, such a mould comprising two mutually joinable mould halves which in their joined state define at least one cavity, and a sprue formed in the mould for passing a melt to each cavity.
In injection moulding of a plastic component, a plastic melt is introduced into a cavity defined by two mould halves of a mould. More specifically, the plastic melt is passed through a channel system extended in the mould, such as a hot runner system, up to an ingate through which the melt enters the cavity.
It will be appreciated that the ingate causes a surface defect in the component. The location and design of the ingate is thus dependent on the surface requirements that are placed on the completed component.
In some cases it is desirable for the surface defect caused by the ingate to be as small as possible and located on an inside or rear side of the component. The ingate is placed on a side of the cavity that corresponds to the inside or rear side of the component. Examples of components where it is most important for such minor surface defects to appear on a side that is not exposed to the surroundings are high status products, such as casings for mobile phones.
This can be achieved, for example, by placing a suitably designed ingate directly at the end of the hot runner system and simultaneously forming the cavity in such manner that the inside of the component faces the hot runner system.
This solution, however, suffers from a number of drawbacks. For removing the completed component from the mould, ejectors are normally used, which when operated are caused to engage the component. Ejectors are normally arranged in such manner as to engage the inside of the component so as to guarantee an outside without surface defects caused by the ejectors. To render this possible in the solution described above, the ejectors must thus be arranged in the same mould half as the hot runner system. The hot runner system, which is usually arranged in a fixed first mould half, is however in most cases fairly bulky, and therefore it may be difficult to find space for the ejectors in the same mould half as said hot runner system. Moreover, it may be difficult to arrange the mechanism that drives the ejectors in the fixed mould half.
It is thus common to arrange the ejectors in a movable second mould half and, if the ejectors are to engage an inside of the component, consequently it is necessary to conduct the melt from the hot runner system, on to the second mould half and then on to the ingate which leads to the cavity on a side corresponding to the inside of the component.
A prior art method of accomplishing this is illustrated in FIGS. 1-4. According to this solution, an insert 101 is arranged in the second mould half 102. The insert 101 defines two interconnected channels 103, 104. A first 103 of the channels forms an extension of an ejector channel 105 formed in the second mould half 102. A second 104 of the channels forms a trunk-like intake and extends arcuately in and under the cavity 106 and leads to an ingate positioned on a side of the cavity 106 corresponding to the inside of the component.
In said ejector channel 105, a first ejector 110 is arranged, which thus forms a demarcation of the first channel 103.
When producing a component 107, a melt 108 is introduced through a hot runner system 109 into the two channels 103, 104, which is clearly to be seen in FIG. 2.
That part of the melt 108 which is introduced into the second channel 104 fills the cavity 106 through the ingate 111.
The injection moulded component 107 is then cooled, after which the mould halves 102, 112 are divided and the ejectors 110, 113 operated, as shown in FIG. 3.
By operation of the ejectors 110, 113 they are caused to engage the component 107 for ejection.
That part of the melt 107 which fills the two channels 103, 104 forms a sprue slug 114. When operating the ejectors 110, 113, the first ejector 110 will engage the cylindrical part 115 of the sprue slug 114 which fills the first channel 103. By ensuring that the length of the first channel 103 is greater than the length of the arcuate portion 116 of the second channel 104 it is ensured that the trunk-like part 117 of the sprue slug 114 which fills said arcuate portion 116 is released from the component 107 and extracted from the arcuate portion 116.
FIG. 4 illustrates both the component 107 and the sprue slug 114 in the ejected state.
This prior art method, however, suffers from a number of drawbacks. First, it causes a considerable amount of waste material in the form of sprue slugs 114. The insert 101 must be made of two pieces which in their joined state define said channels 103, 104. The reason for this is that it would otherwise not be possible to provide said arcuate portion 104 of the second channel 104. Finally, there is a risk of the trunk-like part 117 of the sprue slug 114 being broken off from the cylindrical part 115. In particular it will be impossible to use non-elastic plastics in injection moulding, such as thermosetting plastics or reinforced plastics.
There is thus a need for an improved method of producing components which have a minor surface defect caused by an ingate on their insides or rear sides.
In view of that stated above, an object of the present invention is to provide an improved mould for producing plastic components.
Another object is to provide such a mould for producing components which have a minor surface defect caused by an ingate on a side that is not exposed to the surroundings.
It is also an object of the invention to provide an injection moulding assembly having such a mould.
According to the present invention, the above objects and also other objects that will appear from the following description are achieved by a mould having the features stated in claim 1 and an injection moulding assembly having the features stated in claim 10. Preferred embodiments of the mould are evident from claims 2-9.
More specifically, according to the present invention a mould is provided, comprising two mutually joinable mould halves which in their joined state define at least one cavity and a sprue formed in the mould for passing a melt to each cavity, said mould being characterised in that the sprue comprises an ingate portion connecting to each cavity and formed in a first ejector of an ejector means.
This results in an improved mould which allows production of components having a minor surface defect caused by an ingate and located on an inside or rear side of each component. This is achieved thanks to the fact that the sprue that passes the melt to the cavity comprises an ingate portion contained in one of the ejectors. As a result, the need for complicated inserts in the mould is eliminated while at the same time it will be possible to injection mould components of reinforced plastics or thermosetting plastics.
According to a preferred embodiment of the inventive mould, the ejector means comprises additional ejectors, each ejector being arranged in an ejector channel formed in said second mould half. Said first ejector is advantageously adapted to perform its ejecting motion with a time delay in relation to the other ejectors. This makes it possible to accomplish release of a sprue slug from the completed component during the initial ejecting process. For instance, this can be achieved by the mould comprising press pins which are insertable into said ejector channels for engaging the ejectors. The press pin which engages said first ejector is arranged in such manner that its engagement is delayed in terms of time compared with the engagement of the other press pins with associated ejectors.
According to another preferred embodiment, the ingate portion contained in said first ejector comprises an inlet in the circumferential surface of the ejector and an ingate-forming outlet in an end face of the ejector. Said inlet is advantageously axially displaced from said end face.
The ingate portion has preferably a cross-sectional area tapering towards the ingate-forming outlet.
According to yet another embodiment, the channel also comprises a connecting portion which extends from a parting plane between the mould halves of the mould to said ingate portion.
Further according to the present invention, an injection moulding assembly is provided, comprising a mould at stated above.
A preferred embodiment of the present invention will now be described by way of example and with reference to the accompanying drawings.